Heat engines are used to convert heat or thermal energy into useful mechanical work and are often used in power generation plants. One example of a portion of a heat engine is an expander-generator system which generally includes an expander (e.g., a turbine) rotatably coupled to a generator or other power generating device via a common shaft. As the working fluid expands in the expander, the shaft is forced into rotational movement which excites a flow of electrons in the windings of the generator such that a flow of electrical power ensues. The electrical power may then be harnessed and used as useful work.
Most power plant expander-generators are based on the Rankine cycle and obtain high temperature/pressure working fluids through the combustion of coal, natural gas, oil, and/or nuclear fission. Typical working fluids for Rankine cycles include water (steam) and certain hydrocarbon (organic) fluids. Recently, however, due to perceived benefits in terms of hardware compactness, efficiency, and heat transfer characteristics, there has been considerable interest in using super-critical carbon dioxide (ScCO2) as a working fluid for certain expander-generator applications. Notable among such applications are nuclear, solar, and waste heat energy conversion cycles.
Although ScCO2 has several remarkable advantages as a process fluid, its small ratio of specific heat makes its use in waste heat recovery cycles problematic. The small specific heat value results in a small temperature drop in the ScCO2 gas through a typical heat cycle pressure/expansion process. This relatively small temperature drop can limit the amount of waste heat energy recovery possible with simple Rankine (and Brayton) cycles, especially for applications with waste heat streams of high initial gas temperatures.
One solution to this problem has been to use multiple, separately-cascaded heat cycles that operate with different heat input temperatures in order to handle a larger range of waste heat stream temperatures. This solution, however, requires two or more separate gas expanders that operate over substantially the same pressure ratio, but at differing temperatures. Using plural gas expanders increases operational and capital costs and decreases system reliability since multiple pieces of equipment can potentially fail or cause problems.
What is needed, therefore, is a practical approach to combining two or more expansion processes into a single, compact, and effective turbomachine suitable for such cascaded cycle applications.